Define UMA events for interesting power adapter situations.

This adds events to the Platform.CrOSEvent histogram to record
specific situations we're interested in.  The user of the events
will be thermal.sh in the daisy overlay.

BUG=chrome-os-partner:23973
TEST=tested manually and observed samples in about:histograms

Change-Id: I1845ce1f90f5e77ecdc294834671328ce13c827b
Reviewed-on: https://chromium-review.googlesource.com/176367


Commit-Queue: Luigi Semenzato <semenzato@chromium.org>
Tested-by: Luigi Semenzato <semenzato@chromium.org>
Reviewed-by: Vincent Palatin <vpalatin@chromium.org>